1. Field of the Invention.
This invention relates generally to wrist pins used to connect a piston to a connecting rod, and, more specifically, to aluminum wrist pins.
2. Prior Art.
In a great number of applications, the vast majority of reciprocating piston assembly parts are constructed of aluminum. This is true in internal combustion engines, pumps, reciprocating compressors and other applications. Aluminum provides a lighter, and more efficient, alternative to steel or cast iron. However, it has been universally accepted for many years that the wrist pin, which connects the piston and connecting rod, must at least have an outer cylindrical surface constructed of steel. Thus, wrist pins have historically been constructed of steel or compositions of steel and aluminum, or steel and plastic, always having an outer cylindrical surface of steel.
A serious problem results when a steel wrist pin is placed in a bore of an aluminum piston. The coefficient of expansion for aluminum greatly differs from that of steel. Therefore, as the piston assembly heats during use, the bore of the piston enlarges at a rate greater than that of the steel surface of the wrist pin, causing the pin to become loose in the bore. Over time, damage to the piston, pin and/or the cylinder wall results from this loose condition. While, ideally, it would be beneficial to manufacture the pin and piston of the same material, it has been accepted that this is not possible, given the softness and lack of bearing capacity of aluminum. In an attempt to reduce the weight of steel wrist pins, tubular steel wrist pins have been employed. Tubular steel wrist pins are manufactured by boring a solid steel wrist pin. During the manufacturing process, some burrs and other contaminants will travel within the bore of the tubular steel pin, and ultimately end up in the assembled machinery, possibly causing long term damage of the machinery. The bores of tubular steel pins have been filled with aluminum or polymeric material, but such practices dramatically increase the cost of manufacture. Additionally, in order to minimize the damage caused by steel pins, steel pins are manufactured to very tight tolerances, also increasing the cost of manufacture.